Devulcanization of cured rubber

ABSTRACT

This invention is based upon the unexpected discovery that cured rubber can be devulcanized by heating it to a temperature of at least about 150° C. under a pressure of at least about 3.4×10 6  Pascals in the presence of 2-butanol. The molecular weight of the rubber can be maintained at a relatively high level if the devulcanization is carried out in the presence of the 2-butanol at a temperature of no more than about 300° C. This devulcanization technique does not significantly change the microstructure of the rubber and it can accordingly be used in the same types of applications as was the original rubber. In other words, the devulcanized rubber can be recompounded and recurred into useful articles in substantially the same way as was the original rubber. This invention more specifically discloses a process for devulcanizing cured rubber into devulcanized rubber that is capable of being recompounded and recurred into useful rubber products, said process comprising (1) heating the cured rubber to a temperature which is within the range of about 150° C. to about 300° C. under a pressure of at least about 3.4×10 6  Pascals in the presence of 2-butanol to devulcanize the cured rubber into the devulcanized rubber thereby producing a slurry of the devulcanized rubber in the 2-butanol; and (2) separating the devulcanized rubber from the 2-butanol.

BACKGROUND OF THE INVENTION

Millions of used tires, hoses, belts and other rubber products arediscarded annually after they have been worn-out during their limitedservice life. These used rubber products are typically hauled to a dumpbecause there is very little use for them after they have served theiroriginal intended purpose. A limited number of used tires are utilizedin building retaining walls, as guards for protecting boats and similarthings where resistance to weathering is desirable. However, a fargreater number of tires, hoses and belts are simply discarded.

The recycling of cured rubber products has proven to be an extremelychallenging problem. This problem associated with recycling cured rubberproducts (such as, tires, hoses and belts) arises because, in thevulcanization process, the rubber becomes crosslinked with sulfur. Aftervulcanization, the crosslinked rubber becomes thermoset and cannot bereformed into other products. In other words, the cured rubber cannot bemelted and reformed into other products like metals or thermoplasticmaterials. Thus, cured rubber products cannot be simply melted andrecycled into new products.

Since the discovery of the rubber vulcanization process by CharlesGoodyear in the nineteenth century, there has been interest in therecycling of cured rubber. A certain amount of cured rubber from tiresand other rubber products is shredded or ground to a small particle sizeand incorporated into various products as a type of filler. Forinstance, ground rubber can be incorporated in small amounts intoasphalt for surfacing roads or parking lots. Small particles of curedrubber can also be included in rubber formulations for new tires andother rubber products. However, it should be understood that therecycled rubber serves only in the capacity of a filler because it waspreviously cured and does not co-cure to an appreciable extent to thevirgin rubber in the rubber formulation.

Various techniques for devulcanizing cured rubber have been developed.Devulcanization offers the advantage of rendering the rubber suitablefor being reformulated and recurred into new rubber articles if it canbe carried out without degradation of the rubber. In other words, therubber could again be used for its original intended purpose. However,none of the devulcanization techniques previously developed have provento be commercially viable.

U.S. Pat. No. 4,104,205 discloses a technique for devulcanizingsulfur-vulcanized elastomer containing polar groups which comprisesapplying a controlled dose of microwave energy of between 915 MHz and2450 MHz and between 41 and 177 watt-hours per pound in an amountsufficient to sever substantially all carbon-sulfur and sulfur--sulfurbonds and insufficient to sever significant amounts of carbon--carbonbonds.

U.S. Pat. No. 5,284,625 discloses a continuous ultrasonic method forbreaking the carbon-sulfur, sulfur--sulfur and, if desired, thecarbon--carbon bonds in a vulcanized elastomer. Through the applicationof certain levels of ultrasonic amplitudes in the presence of pressureand optionally heat, it is reported that cured rubber can be brokendown. Using this process, the rubber becomes soft, thereby enabling itto be reprocessed and reshaped in a manner similar to that employed withpreviously uncured elastomers.

U.S. Pat. No. 5,602,186 discloses a process for devulcanizing curedrubber by desulfurization, comprising the steps of: contacting rubbervulcanizate crumb with a solvent and an alkali metal to form a reactionmixture, heating the reaction mixture in the absence of oxygen and withmixing to a temperature sufficient to cause the alkali metal to reactwith sulfur in the rubber vulcanizate and maintaining the temperaturebelow that at which thermal cracking of the rubber occurs, therebydevulcanizing the rubber vulcanizate. U.S. Pat. No. 5,602,186 indicatesthat it is preferred to control the temperature below about 300° C., orwhere thermal cracking of the rubber is initiated.

SUMMARY OF THE INVENTION

By utilizing the process of this invention, cured rubber can bedevulcanized using a simple technique without the need for microwaves,ultrasonic waves or an alkali metal. In other words, the cured rubbercan be devulcanized in the absence of microwaves, ultrasonic waves or analkali metal. The employment of the process of this invention alsopreserves the original microstructure of the rubber and allows for it tomaintain a relatively high molecular weight. Thus, the process of thisinvention primarily breaks sulfur--sulfur bonds and/or carbon-sulfurbonds rather than carbon--carbon bonds.

This invention is based upon the unexpected discovery that cured rubbercan be devulcanized by heating it to a temperature of at least about150° C. under a pressure of at least about 3.4×10⁶ Pascals in thepresence of 2-butanol. The molecular weight of the rubber can bemaintained at a relatively high level if the devulcanization is carriedout in the presence of the 2-butanol at a temperature of no more thanabout 300° C. This devulcanization technique does not significantlychange the microstructure of the rubber and it can accordingly be usedin the same types of applications as was the original rubber. In otherwords, the devulcanized rubber can be recompounded and recurred intouseful articles in substantially the same way as was the originalrubber.

This invention more specifically discloses a process for devulcanizingcured rubber into devulcanized rubber that is capable of beingrecompounded and recurred into useful rubber products, said processcomprising (1) heating the cured rubber to a temperature which is withinthe range of about 150° C. to about 300° C. under a pressure of at leastabout 3.4×10⁶ Pascals in the presence of 2-butanol to devulcanize thecured rubber into the devulcanized rubber thereby producing a slurry ofthe devulcanized rubber in the 2-butanol and (2) separating thedevulcanized rubber from the 2-butanol.

This invention also reveals a process for devulcanizing cured rubberinto devulcanized rubber that is capable of being recompounded andrecurred into useful rubber products, and for extracting thedevulcanized rubber from the cured rubber, said process comprising (1)heating the cured rubber to a temperature which is within the range ofabout 150° C. to about 300° C. under a pressure of at least about3.4×10⁶ Pascals in 2-butanol to devulcanize the cured rubber into thedevulcanized rubber thereby producing a mixture of solid cured rubber,solid devulcanized rubber and a solution of the devulcanized rubber inthe 2-butanol, (2) removing the solution of the devulcanized rubber fromthe solid cured rubber and the solid devulcanized rubber, (3) coolingthe solution of the devulcanized rubber in the 2-butanol to atemperature of less than about 100° C. and (4) separating thedevulcanized rubber from the 2-butanol.

DETAILED DESCRIPTION OF THE INVENTION

Virtually any type of sulfur-cured rubber can be devulcanized byutilizing the process of this invention. For instance, it can be used todevulcanize natural rubber, synthetic polyisoprene rubber, polybutadienerubber, styrene-butadiene rubber, isoprene-butadiene rubber,styrene-isoprene rubber, styrene-isoprene-butadiene rubber, nitrilerubber, carboxylated nitrile rubber, bromobutyl rubber, chlorobutylrubber and the like. The technique of this invention can also be used todevulcanize blends of various types of rubbers.

The devulcanization process of this invention can be carried out bysimply heating the cured rubber in the presence of 2-butanol to atemperature of at least about 150° C. under a pressure of at least about3.4×10⁶ Pascals (Pa). To increase the rate of the devulcanizationprocess, the cured rubber will typically be cut, milled or ground to arelatively small particle size. It is normally preferred for thetemperature to be no more than about 300° C. to minimize the level ofpolymer degradation. In other words, if the devulcanization process isconducted at a temperature of no more than about 300° C., thesulfur--sulfur and/or carbon-sulfur bonds in the cured rubber can bebroken preferentially to the carbon--carbon bonds in the rubber. Thus,by carrying out the devulcanization process at a temperature of 300° C.or less, the molecular weight of the rubber can be maintained at a highlevel. For this reason, the devulcanization process will typically beconducted at a temperature which is within the range of about 150° C. toabout 300° C.

It is normally preferred for the devulcanization process to be carriedout at a temperature which is within the range of about 200° C. to about280° C. The most preferred devulcanization temperatures are within therange of about 230° C. to about 260° C. The pressure employed willtypically be within the range of about 3.4×10⁶ Pascals (500 lbs/in²) toabout 3.4×10⁷ Pascals (5000 lbs/in²). It is normally preferred toutilize a pressure which is within the range of about 6.9×10⁶ Pascals(1000 lbs/in²) to about 2.8×10⁷ Pascals (4000 lbs/in²). It is generallymost preferred to utilize a pressure which is within the range of about1.7×10⁷ Pascals (2500 lbs/in²) to about 2.4×10⁷ Pascals (3500 lbs/in²).It is normally preferred for the cured rubber being devulcanized to beemersed in a bath of 2-butanol. In any case, it is important to protectthe devulcanized rubber from oxygen during the process. In some cases,it will be desirable to conduct the process under an inert gasatmosphere, such as nitrogen.

After the devulcanization has been completed, the devulcanized rubber isseparated from the 2-butanol. Since the devulcanized rubber is somewhatsoluble in the 2-butanol at elevated temperatures, the separation willtypically be carried out at a temperature of less than about 100° C. Thedevulcanized rubber can be recovered from the 2-butanol utilizingconventional techniques for separating solids from liquids. Forinstance, the devulcanized rubber can be recovered from the 2-butanoland other solid residue (such as, carbon black, silica and metals) bydecantation, filtration, centrification and the like.

Since the devulcanized rubber is somewhat soluble in 2-butanol at hightemperatures, it is possible to extract the devulcanized rubber fromcured rubber and other solid residue using 2-butanol as the solvent.This involves (1) heating the cured rubber to a temperature which iswithin the range of about 150° C. to about 300° C. under a pressure ofat least about 3.4×10⁶ Pascals in 2-butanol to devulcanize the curedrubber into the devulcanized rubber thereby producing a mixture of solidcured rubber, solid devulcanized rubber, in most cases additional solidresidue, such as fillers (carbon black, silica, clay, and the like)and/or metals, and a solution of the devulcanized rubber in the2-butanol, (2) removing the solution of the devulcanized rubber from thesolid cured rubber and the solid devulcanized rubber, (3) cooling thesolution of the devulcanized rubber in the 2-butanol to a temperature ofless than about 100° C. and (4) separating the devulcanized rubber fromthe 2-butanol.

The devulcanized rubber made by the process of this invention can berecompounded and recurred into useful rubber products, such as tires,hoses and belts. The weight average molecular weight of the rubber canbe maintained at a high level of over 100,000 and typically over150,000. In some cases, a weight average molecular weight of over200,000 can be maintained. The devulcanization technique of thisinvention does not significantly change the microstructure of the rubberand it can accordingly be used in the same types of applications as wasthe original rubber. In other words, the devulcanized rubber can berecompounded and recurred into useful articles in substantially the sameway as was the original rubber.

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, parts and percentages are givenby weight.

EXAMPLES 1-10

In this series of experiments, cured styrenebutadiene rubber (SBR)containing 23.5 percent bound styrene was devulcanized in a variousalcohols, including methanol, ethanol, 1-butanol, 1-propanol, 2propanol,2-butanol, isobutyl alcohol, 4-methyl-2pentanol and 1-pentanol. Thealcohol was injected into a Hewlett-Packard 5890A gas chromatograph at apressure of 2.1×10⁷ Pascals (3000 lbs/in²) with an ISCO LC-5000 syringepump. The Hewlett-Packard 5890A gas chromatograph was not used in thecapacity of a chromatographic instrument. The chromatograph was usedsolely to provide a temperature controllable environment. In otherwords, the chromatograph was used in the capacity of a heating oven. Thesample vessel in the gas chromatograph contained about 0.55 grams ofcured SBR samples which were devulcanized and extracted by the alcoholthat passed through the sample vessel which was inline with an all-metalflow path.

In the procedure used, the SBR samples were initially heated to atemperature of 150° C. and maintained at that temperature under staticconditions for 10 minutes in the alcohol which was, of course, under thepressure of 2.1×10⁷ Pascals (3000 lbs/in²). Then, the alcohol wasallowed to flow through the system at a flow rate of 1-2 ml per minuteat a temperature of 150° C. for 20 minutes with the alcohol exiting thechromatograph being collected and the amount of devulcanized SBR thatwas extracted being measured.

Then, the temperature of the sample chamber was increased to 200° C. andwas maintained at that temperature under static conditions for 10additional minutes with the alcohol still being maintained at a pressureof 2.1×10⁷ Pascals (3000 lbs/in²). Then, the alcohol was again allowedto flow through the system at a flow rate of 1-2 ml per minute at atemperature of 200° C. for 20 minutes with the alcohol exiting thechromatograph being collected and with the amount of devulcanized SBRthat was extracted being measured.

Then, the temperature of the sample chamber was increased to 250° C. andwas maintained at that temperature under static conditions for 10additional minutes with the alcohol being maintained at a pressure of2.1×10⁷ Pascals (3000 lbs/in²). Then, the alcohol was again allowed toflow through the system at a flow rate of 1-2 ml per minute at atemperature of 250° C. for 20 minutes with the alcohol exiting thechromatograph being collected and with the amount of devulcanized SBRextracted by the alcohol being measured.

Finally, the temperature of the sample chamber was increased to 300° C.and was maintained at that temperature under static conditions for 10additional minutes with the alcohol being maintained at a pressure of2.1×10⁷ Pascals (3000 lbs/in²). Then, the alcohol was again allowed toflow through the system at a flow rate of 1-2 ml per minute at atemperature of 300° C. for 20 minutes with the alcohol exiting thechromatograph being collected and with the amount of devulcanized SBRextracted by the alcohol being measured.

The cumulative percentage of devulcanized SBR that was extracted fromthe cured SBR sample with each of the alcohols evaluated at 150° C.,200° C., 250° C. and 300° C. is reported in Table I. Example 2 is arepeat of Example 1. Examples 3-10 are comparative examples wherealcohols other than 2-butanol were used for the devulcanization.

                  TABLE I    ______________________________________    Example  Alcohol   150° C.                                200° C.                                      250° C.                                             300° C.    ______________________________________    1        2-butanol 38%      82%   90%    93%    2        2-butanol 40%      70%   85%    92%    3        methanol  2%       3%    4%     7%    4        ethanol   2%       4%    9%     20%    5        1-propanol                       3%       16%   43%    69%    6        2-propanol                       2%       7%    13%    25%    7        1-butanol 4%       19%   57%    86%    8        isobutyl  2%       10%   44%    74%             alcohol    9        1-pentanol                       3%       11%   42%    89%    10       4-methyl-2-                       2%       11%   33%    68%             pentanol    ______________________________________

As can be seen from Table I, 2-butanol was far better than any of theother alcohols evaluated. It was particularly superior at lowertemperatures. In fact, at 200° C., it extracted at least 70 percent ofthe SBR and, at 250° C., it extracted at least 85 percent of the SBR.The utilization of lower temperatures is, of course, desirable becauseless polymer degradation occurs at lower temperatures. The devulcanizedSBR samples that were extracted were determined to have the samemicrostructure as the original SBR samples.

EXAMPLES 11-20

In this series of experiments, the general procedure utilized inExamples 1-10 was repeated except that temperature was held constant at250° C. and the alcohol was allowed to flow continuously at a rate of1-2 ml per minute for 20 minutes at pressure. In this series ofexperiments, 2-butanol was used exclusively as the alcohol for thedevulcanizations. Cured SBR samples that contained no filler, carbonblack, silica or a combination of carbon black and silica weredevulcanized and extracted with the 2-butanol. The SBR had an originalweight average molecular weight of about 400,000. The weight averagemolecular weights of the devulcanized SBR samples recovered are reportedin Table II.

                  TABLE II    ______________________________________    Example    Filler       Molecular Weight*    ______________________________________    11         no filler    181,000    12         no filler    186,000    13         silica       244,000    14         silica       293,000    15         carbon black 197,000    16         carbon black 216,000    17         carbon black/silica                            177,000    18         carbon black/silica                            177,000    ______________________________________     *The molecular weights reported are weight average molecular weights.

As can be seen from Table II, the technique of this invention could beused to devulcanize rubber samples that contained silica, carbon blackor a combination of silica and carbon black. Table II also shows thatthe devulcanization technique of this invention did not greatly reducethe molecular weight of the rubber. Thus, the devulcanization procedureof this invention destroyed sulfur--sulfur bonds and/or carbon-sulfurbonds without destroying a significant number of carbon--carbon bonds inthe rubber.

Variations in the present invention are possible in light of thedescription of it provided herein. While certain representativeembodiments and details have been shown for the purpose of illustratingthe subject invention, it will be apparent to those skilled in this artthat various changes and modifications can be made therein withoutdeparting from the scope of the subject invention. It is, therefore, tobe understood that changes can be made in the particular embodimentsdescribed which will be within the full intended scope of the inventionas defined by the following appended claims.

What is claimed is:
 1. A process for devulcanizing cured rubber intodevulcanized rubber that is capable of being recompounded and recurredinto useful rubber products, said process comprising (1) heating thecured rubber to a temperature which is within the range of about 150° C.to about 300° C. under a pressure of at least about 3.4×10⁶ Pascals inthe presence of 2-butanol to devulcanize the cured rubber into thedevulcanized rubber thereby producing a slurry of the devulcanizedrubber in the 2-butanol and (2) separating the devulcanized rubber fromthe 2-butanol.
 2. A process as specified in claim 1 wherein the slurryof the devulcanized rubber is cooled to a temperature of less than about100° C. before the devulcanized rubber is separated from the 2-butanol.3. A process for devulcanizing cured rubber into devulcanized rubberthat is capable of being recompounded and recurred into useful rubberproducts, and for extracting the devulcanized rubber from the curedrubber, said process comprising (1) heating the cured rubber to atemperature which is within the range of about 150° C. to about 300° C.under a pressure of at least about 3.4×10⁶ Pascals in 2-butanol todevulcanize the cured rubber into the devulcanized rubber therebyproducing a mixture of solid cured rubber, solid devulcanized rubber anda solution of the devulcanized rubber in the 2-butanol, (2) removing thesolution of the devulcanized rubber from the solid cured rubber and thesolid devulcanized rubber, (3) cooling the solution of the devulcanizedrubber in the 2-butanol to a temperature of less than about 100° C. and(4) separating the devulcanized rubber from the 2butanol.
 4. A processas specified in claim 1 wherein step (1) is carried out at a pressurewhich is within the range of about 3.4×10⁶ Pascals to about 3.4×10⁷Pascals.
 5. A process as specified in claim 4 wherein step (1) iscarried out at a temperature which is within the range of about 200° C.to about 280° C.
 6. A process as specified in claim 5 wherein step (1)is carried out at a pressure which is within the range of about 6.9×10⁶Pascals to about 2.8×10⁷ Pascals.
 7. A process as specified in claim 6wherein step (1) is carried out at a temperature which is within therange of about 230° C. to about 260° C.
 8. A process as specified inclaim 7 wherein step (1) is carried out at a pressure which is withinthe range of about 1.7×10⁷ Pascals to about 2.4×10⁷ Pascals.
 9. Aprocess as specified in claim 1 wherein the cured rubber is a blend ofdifferent types of rubbers.
 10. A process as specified in claim 2wherein step (1) is carried out at a pressure which is within the rangeof about 3.4×10⁶ Pascals to about 3.4×10⁷ Pascals.
 11. A process asspecified in claim 10 wherein step (1) is carried out at a temperaturewhich is within the range of about 200° C. to about 280° C.
 12. Aprocess as specified in claim 11 wherein step (1) is carried out at apressure which is within the range of about 6.9×10⁶ Pascals to about2.8×10⁷ Pascals.
 13. A process as specified in claim 12 wherein step (1)is carried out at a temperature which is within the range of about 230°C. to about 260° C.
 14. A process as specified in claim 13 wherein step(1) is carried out at a pressure which is within the range of about1.7×10⁷ Pascals to about 2.4×10⁷ Pascals.
 15. A process as specified inclaim 2 wherein the cured rubber is a blend of different types ofrubbers.
 16. A process as specified in claim 1 wherein the cured rubberis comprised of natural rubber.
 17. A process as specified in claim 1wherein the cured rubber is comprised of polybutadiene rubber.
 18. Aprocess as specified in claim 1 wherein the cured rubber is comprised ofstyrene-butadiene rubber.
 19. A process as specified in claim 1 whereinthe cured rubber is comprised of bromobutyl rubber.
 20. A process asspecified in claim 1 wherein the cured rubber is comprised ofchlorobutyl rubber.
 21. A process as specified in claim 3 wherein step(1) is carried out at a pressure which is within the range of about3.4×10⁶ Pascals to about 3.4×10⁷ Pascals and at a temperature which iswithin the range of about 200° C. to about 280° C.
 22. A process asspecified in claim 3 wherein step (1) is carried out at a pressure whichis within the range of about 6.9×10⁶ Pascals to about 2.8×10⁷ Pascalsand at a temperature which is within the range of about 230° C. to about260° C.